The present invention relates to a method of making agglomerated cemented carbide, cermet or ceramic powder mixtures useful for making cutting tools for metal machining, tools for rock drilling and wear parts.
Cemented carbide or cermet alloys are made of hard constituents from about 5 to about 20 vol-% metal binder phase essentially based on Co and/or Ni. In cemented carbides, the hard constituent is generally carbides whereas in cermets they are nitrides and/or carbonitrides and possibly carbides.
Cemented carbide or cermet bodies are made by powder metallurgical methods of wet milling a powder mixture containing powders forming the hard constituents and binder phase as well as binders and other additives often of a proprietary nature, drying the milled mixture to a ready-to-press powder with good flow properties, pressing the powder in press tools or extruding to bodies of desired shape and finally sintering.
The milling operation produces a slurry which is suitable for subsequent drying. The drying can be performed by spray drying or freeze granulation followed by freeze drying. As a result of the drying process spherical agglomerates of about 0.1 mm diameter are obtained held together by the binder, generally Poly Ethylene Glycol, PEG. This PEG binder is present during pressing but is ultimately removed during sintering.
It is important that the agglomerates have good flow properties to allow an even filling of the press tool and to allow redistribution thereof in the initial phase of the compaction. This is generally obtained by choosing hard agglomerates. Such agglomerates are obtained by using PEG with a high molecular weight. This often leads to porosity problems in the final sintered body. At a certain point in the compaction operation, soft agglomerates are desired in order to secure an even and homogeneous density in the pressed body. This can be obtained by using a PEG with low molecular weight. This gives a pressed body with low porosity but with a less than even density distribution due to the inferior flow properties. When high molecular weight and low molecular weight PEG polymers are blended, a compromise can be achieved, but it is not optimal neither regarding the flow properties nor the density variations in the green body. Optimal would be a binder that has hard properties during the handling of the granules, the filling of the mold and in the green body, but soft properties during the pressing of the material.
Baroplastic is a novel class of materials which is a block copolymer composition capable of being processed by the application of pressure. It is a core-shell polymer of particles with a core of a polymer with soft properties, and a shell of a polymer with hard properties with the size of the particles in the range of from about 50 to about 200 nm. During normal pressure, the core and the shell polymers are not miscible, and hence they stay separate, as core and shell. However, the miscibility of the polymers increases with increasing pressure, and at high pressures the polymers are mixed and get properties in-between the soft core and the hard shell. The increased miscibility at high pressures is reversible, and on lowering the pressure the polymers form a core-shell structure again.
In “Low-temperature processing of “baroplastics” by pressure-induced flow” by J. A. Gonzalez-Leon, M. H. Acar, S.-W. Ryu, A.-V. G. Ruzette and A. Mayes, Nature vol. 426, 424-428, 2003, a process for manufacturing baroplastic polymers are disclosed. The baroplastic polymers obtained low-temperature formability at ambient temperature and could be remolded without degradation.
In “Core-shell polymer nanoparticles for baroplastic processing” by J. A. Gonzalez-Leon, S.-W. Ryu,S. A. Hewlett, S. H. Ibrahim and A. Mayes, Macromolecules, vol 38, 8036-8044, 2005, the properties of baroplastic polymers are further investigated. Especially, the impact of composition, particle size and structure on the mechanical behavior were investigated.
U.S. Pat. No. 6,632,883, herein incorporated by reference in its entirety, discloses block copolymer compositions capable of being processed by the application of pressure. A method for predicting phase diagrams of polymer blends and block copolymers are also disclosed.